This invention relates to improvements in video display apparatus and, more particularly, to a system for stabilizing the display intensity or "color temperature" of a cathode ray tube. The subject matter of this invention is related to subject matter disclosed in copending U.S. application Ser. No. 572,169 and now U.S. Pat. No. 4,012,775 of C. W. Smith, filed of even date herewith and assigned to the same assignee.
Conventional television display systems employing kinescope cathode ray tubes are subject to performance degradation resulting from instabilities in the operating characteristics of the kinescope or the circuits which drive or bias the kinescope. Prior techniques have been developed which serve to stabilize the signals driving a kinescope. For example, the drive voltages applied to the cathodes of a color kinescope can be stabilized using a feedback scheme; e.g., circuitry which periodically senses the drive voltage at input "black" and "white" levels of operation and corrects for deviations from standard reference voltages by gain adjustment. DC voltages applied to the kinescope can also be stabilized by using precise voltage regulation techniques.
There remains, however, the recognized problem of kinescope electron gun drift which manifests itself as a drift in screen color temperature in a three gun color kinescope. As the electron guns age, their generated beam current per unit of applied voltage (which can be considered a transconductance function) varies, the variations being generally non-uniform in the three different guns. This is a cause of noticeable and undesirable drifts in the display screen color.
The major sources of drift are: aging or long term variations caused by a gradual decrease in cathode activity, not necessarily constant or uniform for each cathode; and cathode operating temperature. The relatively long term variations in emission are caused by filament voltage changes and heat build-up in the gun area, generally a function of how many hours a display tube has been operating. Dynamic heating of each gun depends on the ratio of gun currents drawn to provide the colored picture being instantaneously presented. For example, a long persisting mostly red field causes red gun current almost exclusively, thereby causing an unbalanced heating of the red cathode, which changes its emission characteristics to a different degree than the other cathodes, this change remaining until relative cooling occurs.
Cathode thermal current, I.sub. th, is represented by the Dushman equation: EQU I.sub.th = SA.sub.0 T.sup.2 e.sup.-b.sbsp.0.sup./T amperes
where S and A.sub.0 are constants and b.sub.0 = Dushman constant .apprxeq. 11,600.degree. for an oxide coated cathode
The derivative of the natural logarithm of this equation gives the change in emission with respect to temperature change: ##EQU1## The temperature of the CRT cathode is approximately 1,160.degree. K, which yields EQU dI.sub.th/ I.sub.th = 12dT/T
typical ambient temperature variations, such as in a display monitor, are about 40.degree. C, so that the net change of gun current is of the order of EQU 12 .multidot. 40/1,160 .apprxeq. 40%
Therefore, a 1.degree. C change in cathode temperature yields about a 1% change in gun current, if the gun is fixed bias and not near cut-off.
It is an object of the present invention to provide a stabilizing system which overcomes the problems set forth.